Default Include Path and using INCLUDEPATH

I've been trying to add paths to search for headers using the INCLUDEPATH in the .pro file. I understand "INCLUDEPATH += /some/directory" appends to the include paths searched, but from what I am seeing in my project, the path I append is searched before the default ones.

For example, I have "#include <default.h>" which is found in the default paths checked without adding any new paths, but also have "#include <someother.h>", which is not found in the default paths. When I use INCLUDEPATH and append a new directory path, both default.h and someother.h are found in the newly appended directory. I expected the default.h to still be used from the default path though. Am I misunderstanding how the include paths are searched when new ones are appended?

Also, what are the default include paths or where are they defined in QT?